3. Not having an emergency fund.

"A lot of times if people have some debt, they think, 'I should pay off my debt as fast as I can,'” says Sophia Bera, a certified financial planner and founder of Gen Y Planning. But without an emergency fund, your next $1,000 car repair could end up on your credit card.What to do: Set aside $100 or even $50 a paycheck. At a minimum, have an extra $1,000 on hand, and eventually build up a cushion of three to six months of living expenses — in cash, not investments. (For more on establishing emergency savings, read this.